This application claims priority under 35 U.S.C.xc2xa7xc2xa7119 and/or 365 to 9804426-6 filed in Sweden on Dec. 18, 1998; the entire content of which is hereby incorporated by reference.
The present invention relates in general to the field of cellular radio communication and, in particular, to methods and means for determining a handover in a cellular radio communication system.
By xe2x80x9cradio unitxe2x80x9d is meant all portable and non-portable equipment intended for radio communication, like mobile/cellular phones, transceivers, pagers, telex, electronic notebooks, laptops with integrated radios, communicators, computers, routers, tailored microchips or any other electronic equipment using a radio link as a mean of communication. These equipments can be used in any type of radio communication system, such as cellular networks, satellite or small local networks.
Cellular radio communication systems are commonly employed to provide voice and data communications to a plurality of radio units or subscribers.
A cellular radio communication system, e.g. AMPS, NMT, D-AMPS, GSM, DECT and IS-95 (CDMA), generally includes one or more radio units, one or more base stations (e.g. base station transceivers), one or more base station controllers (BSC) and at least one mobile switching centre (MSC). A typical cellular radio communication system may include hundreds of base stations, thousands of radio units and more than one mobile switching centre.
The mobile switching centre and the plurality of base stations associated therewith generally define the cellular radio network.
The cellular radio network which cover a certain geographical area is typically divided into cells or regions, i.e. a part of the geographical area. The cells typically include a base station and the radio units with which the base station is in communication. The cell associated with the particular base station with which a radio unit is communicating is commonly called the serving cell.
Each cell will have allocated to it one or more voice/data and/or traffic channels and in some cases one or more dedicated control channels. These channels are used for communication between the base station and the radio units within the cell. For example, a typical cell of a GSM digital cellular radio network has one digital control channel and 21 voice/data or traffic channels.
Note that xe2x80x9cchannelxe2x80x9d may refer to a specific carrier frequency in an analogue system, e.g. AMPS and NMT, a specific carrier/slot combination in a hybrid TDMA/FDMA system, e.g. GSM, and one or more assigned codes in a CDMA system.
A connection is an established communication path between two points over a determined communication channel.
A need to change the connection between the radio unit and the base station in the serving cell to a new base station in a neighbouring cell can occur due to the mobility of the radio units and poor radio quality on the channels. The process of transferring the connection, e.g. the call, from one cell to another cell is called handover, i.e. to switch the communication on a channel in the serving cell to another channel in a neighbouring cell. The specific neighbouring cell to which the handover is going to be made is commonly called the target cell.
Today the decision to make a handover is mainly made on power-budget calculations where the cell with the best Carrier to Noise ratio (C/N) (the best power-budget) is selected to serve the radio unit. In many situations this measure is not adequate.
Due to the high interference levels (C/I) in today""s mobile networks the communication quality (e.g. bit error rate) cannot be estimated by C/N only. Instead it is essential to base the handover decisions on a combination of the measure of C/N (power budget) and the BER.
The power-budget for a cell can be determined from the received signal strength in a radio unit and its sensitivity or the received signal strength in a base station and the sensitivity of the receiver in the base station.
The U.S. Pat. No. 5,673,307 describes a method, system and an apparatus for handover in a cellular mobile system. A mobile unit measures the BER and signal strength from the current base station and the neighbouring base stations. A handover is performed if the measured signal strength from a neighbouring base station is higher than the measured signal strength from the current base station. If this is not the case, the BER is checked. If the BER is above a certain level a low signal-strength handover-threshold is selected and if the BER is below said level a high signal-strength handover-threshold is selected. A handover is then performed if the signal strength from a neighbouring base station is higher than the signal-strength handover-threshold.
The European patent application EP 0530165 describes a method, system and an apparatus for call set-up and handover in a cellular mobile radio system. The signal strength from a current base station and neighbouring base stations are measured. If the signal strength from a neighbouring base station is higher than the signal strength from the current base station a handover is performed. The BER is checked and if there are any bit errors a counter is incremented. If the counter has reached a certain value and if a certain time out limit is not exceeded a handover to a new channel in the same base station is performed. If the BER is equal to zero, new comparisons with signal strengths from different base stations are performed and a handover is made either to a new base station or to a new channel in the same base station.
The UK patent application GB 2297 885 describes a method and an apparatus for channel estimation in a cellular mobile radio system. An average value during a certain time period of the signal strength and the BER is used to estimate if the quality on a radio channel is decreasing. A channel change request is generated if the quality is estimated to be degraded.
As will be seen herein, each of the methods and means disclosed in these patents is of different types and construction than the methods and means of the present invention.
The present invention meets a problem related to handover procedures.
The problem occurs when a handover is to be performed on a connection based on the received signal strength only and irrespective of the communication quality of the connection.
In light of the foregoing, a primary object of the present invention is to provide methods and means for determining a handover where the communication quality has the major influence on the handover decision.
Another object of the present invention is to provide methods and means for gradual and flexible handover performance in a shifting radio environment.
In a method according to the present invention power-budgets for the serving and neighbouring cells are calculated. The power-budgets for the neighbouring cells are then adjusted by specific quality adjustment values that are determined by the communication-quality of the cells. A handover decision is then dependent on the values of the adjusted power-budgets of the neighbouring cells.
According to one embodiment of the method the signal strength in the serving cell and from neighbouring cells are measured. The communication-quality in the serving cell is also measured. Power-budgets are calculated and a number of ranking values are determined for the cells. New ranking values for the neighbouring cells are calculated by using communication-quality adjustment values. A handover is determined to be performed to a specific neighbouring cell if the new ranking value for said specific neighbouring cell exceeds the ranking value in the serving cell.
The inventive method is therewith characterised as it appears from the appended claim 1.
An arrangement for utilising the method according to the present invention is characterised as it appears from the appended claim 12.
An advantage with the present invention is that it is possible to handle a number of different levels of bad quality on a connection.
Another advantage is that it is possible to improve the quality of a connection by letting a radio unit be in contact with the base station that offers the best communication-quality and not only the base station offering the best Carrier to Noise ration.
Yet another advantage is that the number of handovers can be reduced.
Still another advantage is that neighbouring cells with a good communication quality can be favoured by early handovers and neighbouring cells with a bad communication quality can be disfavoured by late handovers.